如何避免 c# File.ReadLines First() 锁定文件

Question

我不想在任何时候阅读整个文件，我知道这个问题有答案，我想要 o 阅读第一行或最后一行。

我知道我的代码锁定了它正在读取的文件，原因有两个 1) 当我使用此代码运行我的小应用程序时，写入文件的应用程序会间歇性崩溃，但当我不运行此代码时它永远不会崩溃！ 2）有几篇文章会告诉你File.ReadLines会锁定文件。

有一些类似的问题，但该答案似乎涉及读取整个文件，这对于大文件来说很慢，因此不是我想要做的。我大部分时间只阅读最后一行的要求与我所阅读的内容相比也是独一无二的。

我想知道如何阅读第一行（标题行）和最后一行（最后一行）。我不想在代码中的任何时候读取所有行，因为这个文件可能会变得很大并且读取整个文件会变得很慢。

我知道

line = File.ReadLines(fullFilename).First().Replace("\"", "");

...与...相同

FileStream fs = new FileStream(@fullFilename, FileMode.Open, FileAccess.Read, FileShare.Read);

我的问题是，如何在不以任何方式锁定它的情况下重复读取可能被另一个应用程序写入的文件的第一行和最后一行。我无法控制写入文件的应用程序。它是一个可以随时附加的数据日志。我以这种方式收听的原因是该日志可以连续几天附加。我想在我自己的 c# 程序中查看此日志中的最新数据，而无需等待日志写入完成。

我的代码调用阅读/听力函数...

    //Start Listening to the "data log"
    private void btnDeconstructCSVFile_Click(object sender, EventArgs e)
    {
        MySandbox.CopyCSVDataFromLogFile copyCSVDataFromLogFile = new MySandbox.CopyCSVDataFromLogFile();
        copyCSVDataFromLogFile.checkForLogData();
    }

我的听力课。现在它只是将数据添加到 2 个泛型列表中......

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using MySandbox.Classes;
using System.IO;

namespace MySandbox
{
    public class CopyCSVDataFromLogFile
    {
        static private List<LogRowData> listMSDataRows = new List<LogRowData>();
        static String fullFilename = string.Empty;
        static LogRowData previousLineLogRowList = new LogRowData();
        static LogRowData logRowList = new LogRowData();
        static LogRowData logHeaderRowList = new LogRowData();
        static Boolean checking = false;

        public void checkForLogData()
        {
            //Initialise
            string[] logHeaderArray = new string[] { };
            string[] badDataRowsArray = new string[] { };

            //Get the latest full filename (file with new data) 
            //Assumption: only 1 file is written to at a time in this directory.
            String directory = "C:\\TestDir\\";
            string pattern = "*.csv";
            var dirInfo = new DirectoryInfo(directory);
            var file = (from f in dirInfo.GetFiles(pattern) orderby f.LastWriteTime descending select f).First();
            fullFilename = directory + file.ToString(); //This is the full filepath and name of the latest file in the directory!

            if (logHeaderArray.Length == 0)
            {
                //Populate the Header Row
                logHeaderRowList = getRow(fullFilename, true);
            }

            LogRowData tempLogRowList = new LogRowData();

            if (!checking)
            {
                //Read the latest data in an asynchronous loop
                callDataProcess();
            }
        }

        private async void callDataProcess()
        {
            checking = true;                        //Begin checking
            await checkForNewDataAndSaveIfFound();
        }

        private static Task checkForNewDataAndSaveIfFound()
        {
            return Task.Run(() =>   //Call the async "Task"
            {
                while (checking)    //Loop (asynchronously)
                {
                    LogRowData tempLogRowList = new LogRowData();

                    if (logHeaderRowList.ValueList.Count == 0)
                    {
                        //Populate the Header row
                        logHeaderRowList = getRow(fullFilename, true);
                    }
                    else
                    {
                        //Populate Data row
                        tempLogRowList = getRow(fullFilename, false);

                        if ((!Enumerable.SequenceEqual(tempLogRowList.ValueList, previousLineLogRowList.ValueList)) &&
                            (!Enumerable.SequenceEqual(tempLogRowList.ValueList, logHeaderRowList.ValueList)))
                        {
                            logRowList = getRow(fullFilename, false);
                            listMSDataRows.Add(logRowList);
                            previousLineLogRowList = logRowList;
                        }
                    }

                    //System.Threading.Thread.Sleep(10);  //Wait for next row.
                }
            });
        }

        private static LogRowData getRow(string fullFilename, bool isHeader)
        {
            string line;
            string[] logDataArray = new string[] { };
            LogRowData logRowListResult = new LogRowData();

            try
            {
                if (isHeader)
                {
                    //Asign first (header) row data.
                    //Works but seems to block writting to the file!!!!!!!!!!!!!!!!!!!!!!!!!!!
                    line = File.ReadLines(fullFilename).First().Replace("\"", "");    
                }
                else
                {
                    //Assign data as last row (default behaviour).
                    line = File.ReadLines(fullFilename).Last().Replace("\"", "");
                }

                logDataArray = line.Split(',');

                //Copy Array to Generics List and remove last value if it's empty.
                for (int i = 0; i < logDataArray.Length; i++)
                {
                    if (i < logDataArray.Length)
                    {
                        if (i < logDataArray.Length - 1)
                        {
                            //Value is not at the end, from observation, these always have a value (even if it's zero) and so we'll store the value.
                            logRowListResult.ValueList.Add(logDataArray[i]);
                        }
                        else
                        {
                            //This is the last value
                            if (logDataArray[i].Replace("\"", "").Trim().Length > 0)
                            {
                                //In this case, the last value is not empty, store it as normal.
                                logRowListResult.ValueList.Add(logDataArray[i]);
                            }
                            else { /*The last value is empty, e.g. "123,456,"; the final comma denotes another field but this field is empty so we will ignore it now. */ }
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                if (ex.Message == "Sequence contains no elements")
                { /*Empty file, no problem. The code will safely loop and then will pick up the header when it appears.*/ }
                else
                {
                    //TODO: catch this error properly
                    Int32 problemID = 10;   //Unknown ERROR.
                }
            }

            return logRowListResult;
        }
    }
}

Answer 1

我在其他问题的组合中找到了答案。一个解释如何从文件末尾读取的答案，我对其进行了调整，以便它只能从文件末尾读取 1 行。另一个解释了如何在不锁定它的情况下读取整个文件（我不想读取整个文件，但未锁定部分很有用）。所以现在您可以在不锁定文件的情况下读取文件的最后一行（如果它包含行尾字符）。对于其他行尾分隔符，只需将我的 10 和 13 替换为行尾字符字节...

将以下方法添加到公共类 CopyCSVDataFromLogFile

private static string Reverse(string str)
{
    char[] arr = new char[str.Length];
    for (int i = 0; i < str.Length; i++)
        arr[i] = str[str.Length - 1 - i];
    return new string(arr);
}

并替换这一行...

line = File.ReadLines(fullFilename).Last().Replace("\"", "");

使用此代码块 ...

Int32 endOfLineCharacterCount = 0;
Int32 previousCharByte = 0;
Int32 currentCharByte = 0;
//Read the file, from the end, for 1 line, allowing other programmes to access it for read and write!
using (FileStream reader = new FileStream(fullFilename, FileMode.Open, FileAccess.Read, FileShare.ReadWrite, 0x1000, FileOptions.SequentialScan))
{
    int i = 0;
    StringBuilder lineBuffer = new StringBuilder();
    int byteRead;
    while ((-i < reader.Length) /*Belt and braces: if there were no end of line characters, reading beyond the file would give a catastrophic error here (to be avoided thus).*/
        && (endOfLineCharacterCount < 2)/*Exit Condition*/)
    {
        reader.Seek(--i, SeekOrigin.End);
        byteRead = reader.ReadByte();

        currentCharByte = byteRead;

        //Exit condition: the first 2 characters we read (reading backwards remember) were end of line ().
        //So when we read the second end of line, we have read 1 whole line (the last line in the file)
        //and we must exit now.
        if (currentCharByte == 13 && previousCharByte == 10)
        {
            endOfLineCharacterCount++;
        }

        if (byteRead == 10 && lineBuffer.Length > 0)
        {
            line += Reverse(lineBuffer.ToString());
            lineBuffer.Remove(0, lineBuffer.Length);
        }
        lineBuffer.Append((char)byteRead);

        previousCharByte = byteRead;
    }

    reader.Close();
}